fix bug of being executed by running `rake generate`.

As I have commented on [365f79d](https://github.com/huangbowen521/octopress-syncPost/commit/365f79d375efcacbc1b23bf4c62cdb43216592b4) [plugins/sync_all_posts.rb:L2](https://github.com/huangbowen521/octopress-syncPost/commit/365f79d375efcacbc1b23bf4c62cdb43216592b4#plugins-sync_all_posts-rb-P2), `jekyll` loads every ruby files in the "plugins" folder during its initialization. Therefore, if we put our source code in the "plugins" folder, it will be run when we execute `rake generate`, which is an unexpectedly behavior. So I suggest to change back to the "_custom" folder where we store the ruby files in the old version.